Field of the Invention
This invention relates to audio power amplifiers having a low frequency power amplifier stage constructed with power field-effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube (hereinafter referred to as triode characteristics), this construction permitting to simplify the circuit construction of a drive stage for driving the power amplifier circuit and also obtain production of high quality sounds with high fidelity.
The prior-art audio power amplifiers are roughly classed into two types, namely vacuum tube type and transistor type. The vacuum tube audio power amplifiers include those using pentodes and those using triodes.
Where pentodes are used, high gain and high efficiency can be obtained to obtain high power. In this case, however, problems are encountered in the non-linearility of the pentodes, giving rise to much odd harmonic distortion in the amplifier output, which is undesired from the acoustical standpoint. Also in this case, an output transformer must be used due to the high internal impedance of the pentode. This also contributes to the generation of distortions. Further, the required use of an output transformer is undesirable from the standpoint of the size and weight of the amplifier unit.
Amplifiers using triodes, on the other hand, provide superior lineality to that of the pentode amplifier, so that less negative feedback is required to reduce the distortion factor. Also, in this case the output transformer can be omitted since the internal impedance of the triode is low compared to the pentode. Further, even harmonic distortion constitutes the majority of the distortion component of the amplifier output. Since they have less adverse acoustical effects, sound of superior quality can be reproduced. However, the internal resistance of the triode is still considerably high although it is low compared to that of the pentode. Also, where the output transformer is omitted, it is necessary to provide negative feed-back to a great extent, giving rise to problems in the transient characteristics.
In addition, in the vacuum tube power amplifier using either pentodes or triodes, heater voltage is required, leading to increased power consumption and imposing restrictions upon the size reduction of the amplifier unit.
In the case of the transistor power amplifier, the reduction of size and weight as well as power consumption are possible to realize. However, the transistor has distortion characteristics similar to those of the pentode, mainly consisting of odd harmonic distortions. Therefore, the quality of the sound reproduced is inferior compared to the case of the triode vacuum tube power amplifier, so that the transistor power amplifier is unsuitable as a high-grade audio power amplifier for obtaining high fidelity sound reproduction. The audio power amplifier units described in the prior art are of significantly different construction as compared to the construction defined by the present invention, and particularly as regards the final stage. Thus, relevant prior art audio power amplifier units characteristically do not describe a final stage including power field effect transistors having drain voltage versus drain current characteristics similar to static plate voltage versus plate current characteristics of a triode vacuum tube. Moreover such art does not describe a second stage voltage amplifier circuit having two outputs, reversed in phase and serving as a drive stage. Self-biased field-effect transistor amplifiers are known; however, such units do not include power field-effect transistors having the aforedescribed triode vacuum tube characteristics connected in the manner to be described in detail hereinafter.